1. Field of the Invention
The invention pertains to an x-ray examination apparatus comprising an x-ray source for producing an x-ray image by irradiating an object, an imaging arrangement incorporating an x-ray image intensifier, an image acquisition device for acquiring an image produced on an output screen of the image intensifier and producing a primary video signal, and imaging control means for adjusting imaging parameters.
2. Description of the Related Art
An x-ray examination apparatus of said kind is described in the European patent application EP 0 114 369.
An x-ray examination apparatus as described in the cited reference comprises imaging control means comprising an arithmetic circuit and a diaphragm controller for optimising an adjustment of a diaphragm in front of a television camera tube in order to adapt a dynamic range of brightness-values of the x-ray image to the television camera. To that end a low-dose pre-irradiation is performed prior to x-ray exposure, and a maximum value of a video signal produced by the television camera tube during pre-irradiation is detected. From this maximum value a required diaphragm-value for x-ray exposure is computed by the arithmetic operation circuit, and by way of the diaphragm controller the diaphragm is adjusted such that a maximum output signal at the time of x-ray exposure equals a maximum value which the television camera can output. An x-ray image having a wide dynamic range and having important image information mainly in a lower part of the dynamic range will cause a problem when produced by the known x-ray examination apparatus. Namely, the important image information will be converted into video signals having small signal amplitudes thus having a low signal-to-noise ratio.